Deprecate *_prop entities

[ozxybox]

The entities dynamic_prop, physics_prop, and static_prop should all be deprecated. These entities only lead to mapper and programmer confusion, with mappers not knowing if there is a difference and programmers not realizing the *_prop entities exist. We’re stuck supporting these on the engine for compatibility, but we can deprecate them in Hammer. We should either outright delete their entries or extend FGD to add some type of deprecated tag. Making a deprecated tag could be useful for some of the other legacy entities as well.

https://github.com/StrataSource/FGD/blob/master/fgd/engine/dynamic_prop.fgd
https://github.com/StrataSource/FGD/blob/master/fgd/engine/physics_prop.fgd
https://github.com/StrataSource/FGD/blob/master/fgd/engine/static_prop.fgd

[vrad-exe]

I suggested a "hidden" flag in StrataSource/Engine#691 which could be used for this

[tmob03]

These already are depreciated in the sense that the current system allows, only being compiled in when you want specifically all the engine entities. These aren't in the resulting user FGD